Semiconductor devices are widely used in almost all consumer and home electronic products, as well as in communications, medical, industrial, military, and office products and equipment. Microelectronic semiconductor devices are manufactured from semiconductor wafers. These devices are often just fractions of a micron. This makes these microelectronic devices highly susceptible to performance degradation or even complete failure due to contamination by organic or metal particles. Consequently, cleaning the wafers, to remove contamination, is often a critical step in the manufacturing process.
For many years, wafers were cleaned in typically three or four separate steps using strong acids, such as sulfuric acid, and using strong caustic solutions, such as mixtures of hydrogen peroxide or ammonium hydroxide. Organic solvents have also been used with wafers having metal films. While these methods performed well, they had certain disadvantages, including the high cost of the process chemicals, the relatively long time required to get wafers through the various cleaning steps, high consumption of water due to the need for extensive rinsing between chemical steps, and high disposal costs. As a result, extensive research and development efforts focused on finding better wafer cleaning techniques.
More recently, the semiconductor manufacturing industry began to acknowledge a revolutionary new process for cleaning wafers, using ozone diffused through a thin layer of heated water on the wafers. This ozone diffusion process has proven itself to be highly effective in cleaning contamination and organic films off of wafers, while avoiding many of the disadvantages of the older methods using acids and caustics. The advantages of the ozone diffusion process are that it is fast, requires no expensive and toxic liquid acids or caustics, and operates effectively as a spray process, which greatly reduces water consumption and space requirements.
The ozone diffusion cleaning technique can be performed in various ways. These include spraying water onto the workpiece while injecting ozone into the water, spraying water on the workpiece while delivering ozone to the workpiece, delivering a combination of steam or water vapor and ozone to the workpiece, and applying water, ozone, and sonic energy simultaneously to the workpiece. Spray techniques using water at elevated temperatures have been especially successful at increasing the removal rates of various organic films and contaminants from workpiece surfaces.
Certain metals that are commonly used on semiconductor wafers can corrode when exposed to ozone and heated water. As the process temperatures increase, the chemical reaction rate of all reactions, including metal corrosion, also increases. Dissimilar metals in ohmic contact with each other can also create a galvanic cell potential or electrical interaction which may promote corrosion.
Several methods have been developed to reduce or avoid corrosion. These methods typically include reducing the process temperature and/or using additives that include various corrosion inhibitors. Reducing the temperature is generally undesirable because it slows down the reaction rates of the chemicals acting to remove the organic films or contaminants from the workpiece. Corrosion inhibitors, which generally include additives such as nitrates, silicates, and benzo triazole, have been relatively effective at reducing corrosion on predominantly aluminum films. The application of these inhibitors with the ozone cleaning techniques has allowed use of higher process temperatures, to achieve higher cleaning or strip rates, while substantially controlling corrosion of aluminum surfaces on the wafers.
Still, use of corrosion inhibitors in cleaning semiconductor wafers can be disadvantageous as it involves using an additional chemical or additive, the corrosion inhibitors must be appropriately mixed with the process liquid, and their effectiveness can vary with different metals and other process parameters. Accordingly, there is still a need for a methods for efficiently cleaning a semiconductor wafers using the diffused ozone techniques, while also preventing corrosion of metals, such as copper and aluminum, on the wafers.